As the clean and renewable energy, wind power has witnessed swift development and takes up an ever-growing proportion in the power system. In particular, the grid-forming control technology enables the wind turbine generators to possess such synchronous support capabilities as the active support of system frequency and voltage, along with the adaptability to the weak grid. Nevertheless, there exists a potential worry that the grid-forming wind turbine units may encounter multi-machine instability and result in power oscillations in the circumstance that a large number of wind turbine integrating to the grid in the wind farm with a relatively short electrical distance. Therefore, in this article, a state space model of multiple grid-forming wind turbine is established in the first place with shaft, inductor motor and converter taken into consideration. Then, the eigen mode method was employed to exhibit the weak frequency band, and the participation factor was utilized to analyze the participation links of the weakly damped mode. Thereafter, the influence of key control parameters on the characteristic mode was analyzed by using the root locus method, and the parameter stability region was depicted. At last, the simulation verification was conducted in Matlab/Simulink, validating the effectiveness of the conclusions put forward in this paper.